Implant fixture provided with micro-threads

ABSTRACT

The present invention relates to an implant having a body with at least one generally cylindrical part to be implanted into bone tissue. The cylindrical part is at least partly provided with threads (2) having a height between 0.02 mm and 0.20 mm.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to an element comprising at least onecylindrical part to be implanted into bone tissue.

BACKGROUND TO THE INVENTION

There are two kinds of main systems for endo-osseous dental implantswhich are commonly used today.

One system utilizes fixtures provided with threads which are threadedinto a hole drilled into the jaw-bone. This system can be exemplified bythe so-called Brånemark system™. This system comprises both threadedfixtures which are to be screwed into holes which have been providedwith threads in advance and self-tapping fixtures which are screwed intoa non-pretapped hole drilled in the jaw-bone.

The other system commonly used can be exemplified by the so calledIMZ-implant, which utilizes a cylinder provided with a rough surfaceserving as a fixture and which gently is tapped into a bore-hole in thejaw-bone. The roughness of the surface has no specific orientation.

The threaded fixtures have some important advantages, a major one beinga result of the fact that the main loads in the clinical situation areaxial loads. Threaded implants are very well suited to support axialloads and this may be particularly important in the initial stages ofthe osseointegration process in which it is important that the implantis fully stable and as immovable as possible in the bore-hole. The term"osseo-integration" as coined by Prof Brånemark and his coworkers inGothenburg during the seventies and as used here refers to the closeapposition between bone tissue and implants that for instance may beobtained by using implants made of titanium.

There are however some inherent disadvantages in this construction, oneof the major ones being the time and the care needed to screw aself-tapping implant into a hole. If the hole also has to be providedwith threads in advance, the total period of time needed for theoperation of course will be much greater. Although a conventionalthreaded implant conceivably could be tapped into a hole having almostthe same diameter as the major diameter of the threads, the distance thebone tissue would have to grow into the threads would be excessive andthe time needed for the osseo-integration process would be long.

The rough-surfaced cylindrical implant is very simple to insert and thetime needed for this is short. It may however happen that implantshaving this design gets stuck in the bore-hole before the implant isfully inserted, which may result in an unacceptable trauma to the bonetissue, both if the implant is inserted entirely by force and if theimplant is extracted by force. Both the initial and the final stabilityof the implant will be less than the initial and the final stability ofa threaded implant.

SHORT DESCRIPTION OF THE INVENTIVE CONCEPT

The object of the invention is to provide an implant which combines theadvantages of the two above systems whilst eliminating the disadvantagesthereof.

This object is achieved in that an endo-osseous implant is provided withthe features set forth in the appended main claim.

Preferred embodiments are set forth in the appended dependent claims.

SHORT DESCRIPTION OF THE APPENDED DRAWINGS

FIG. 1 shows an overall view of an implant according to the invention.

FIG. 2 shows a section of the implant in FIG. 1 taken along the the lineI--I.

FIG. 3 shows an end view according to the line II--II in FIG. 2.

FIG. 4 shows a first preferred embodiment of the microthreads and FIG. 5a second preferred embodiment of the microthreads.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

A preferred embodiment of the invention comprises an implant having agenerally cylindrical body 1 for insertion into a bore-hole in bonetissue. The envelope surface of the body 1 is provided with very smallthreads 2, herein called micro-threads since their dimensions are in themicrometer range. These threads will allow the implant to function as ascrew. The forward end or the tip of the screw is provided with threecutting edges 4 in conjunction with chip-collecting cavities 3. A resultof the presence of the chip-collecting cavities is that parts of thecylindrical part are not provided with threads. In view of the way theimplant is intended to function, an area, which is sufficient to allowthe implant to function as a screw, must be provided with threads. Inthis preferred embodiment the tip of the body furthermore is rounded inorder to initially leave some space below the screw for any loose bonechips etc which might impede the full insertion of the screw. The bottomof the bore-hole is normally slightly conically shaped due to the shapeof the drills normally used.

The cutting edges 4 and the chip-collecting cavities 3 will allow thescrew, if necessary, to function as a self-tapping screw for cutting newthreads or for adjusting threads already cut in the tissue.

The other end of the screw is, as is quite conventional in the art,provided with a longitudinal bore for the attachment of an abutment forbridging the soft tissue covering the bone tissue and for the attachmentof a prosthesis. The inner part 7 of the bore is cylindrical andprovided with interior threads 6 and the outer part 5 of the bore isconically flaring in order to accommodate a conically taperingattachment part of an abutment ending in a cylindrical, threaded endportion.

FIGS. 4 and 5 illustrate two different embodiments of the invention. Thethread shown in FIG. 4 is 0.1 mm high and the distance to the adjacentthread (crest to crest) is 0.2 mm. The screw is triple-threaded, whichmeans that the pitch of the thread is 0.6 mm. The reason for thetriple-threaded design rather than a single-threaded design is that thetime needed for screwing the implant into the bore will be less with amultiple-threaded screw. The angle between the flanks of a thread is45°. The threads have a rounded design in order to avoid, or at leastminimize, stress-concentrations in the bone tissue around the threads.

The thread shown in FIG. 5 differs from the thread in FIG. 4 mainly inthat the angle between the flanks is 60° instead of 45°.

Generally, the height of the micro-threads may be within the range of0.02-0.20 mm. In a preferred embodiment the height may vary between 0.02and 0.15 mm, in a more preferred embodiment between 0.05 and 0.15 and ina most preferred embodiment the height is 0.1 mm. The number of threadsis optional but may for instance vary between 1 and 5. In a preferredembodiment the distance between adjacent threads, crest to crest, istwice the height of the threads.

The threads can be regarded as a defined, oriented roughness which is inthe same size range as the prior art non-oriented surface roughness, forinstance of the kind that can be obtained by plasma-spraying, which is aconventional technique for obtaining a surface roughness on implants.

A non-oriented roughness having smaller dimensions, for instanceobtained by blasting techniques, may be superimposed on the threads.

A bio-mechanical study, (Hansson S.: On the role of surface roughnessfor load bearing bone implants: The retention potential of amicro-pitted surface as a function of pit size, pit shape and pitdensity. Thesis, Centre for Biomech., Chalmers Univ. of Technol. andGothenburg Univ., Preprint 1991:4, Gothenburg) has shown that, with aroughness of this size, a retention is obtained which is similar to theretention obtained with more coarse threads.

The implant can be used as both a cylindrical, rough-surfaced implantand as a threaded implant depending on what is suitable from a medicalpoint of view and depending on the preferences of the dentist orsurgeon.

If the implant is used as a cylindrical implant, the implant can belightly tapped into a hole which has the same or almost the samediameter as the major diameter as the implant (the diameter preferablyshould not be larger) in the same way as a conventional implant. Thisnormally can be done relatively quickly. However, should the implant getstuck half-way, which sometimes may happen, the surgeon may choosebetween unscrewing the implant, or screwing the implant fully into thehole. This can be done without exposing the surrounding bone tissue tothe kind of trauma that would have been the result if the implant wereto be extracted forcibly or hammered into place by force.

New bone tissue will rapidly grow into the microthreads due to the lowheight of the threads and a retention which is considerably betterin-the axial direction than in the rotational (tangential) directionwill be obtained relatively quickly. This is of course a result of thefact that the threads are oriented circumferentially. Compared to animplant provided with threads, an implant with a non-oriented surfaceroughness in the size range in question will not offer the sameretention area (i. e. the area which is interlocking with the bonetissue) perpendicularly to the axial direction and consequently will notoffer the same degree of retention.

The design of the implant according to the invention will also permit avery gentle insertion of the implant in the upper jaw, which may be moresensitive than the lower jaw. This is due to the fact that themicro-threaded implant does not require any extensive thread-cuttingoperations in the sometimes relatively fragile bone tissue in the upperjaw and either can be inserted as a self-tapping screw with a minimum ofcutting action or, more importantly, can be just pushed or gently tappedinto a bore-hole in the upper jaw.

A further advantage of the invention is that also small, narrow implantswill have a maximal stiffness or rigidity which may be important in viewof a correct transfer of the main, axial loads to the surrounding bonetissue, since the the threads will form a comparatively small portion ofthe entire cross-section of the implant.

As mentioned above, the implant also can be used in the same way as aconventional screw-threaded implant, in which case the hole bored in thebone tissue should have a diameter corresponding to the minor diameterof the thread or somewhat larger. In the latter case, the force neededto cut the threads in the bone tissue will be less.

It should be noted that the invention can be varied in many ways withinthe scope of the appended claims. It should for instance be emphasizedthat the invention is not limited to dental implants and that theinvention could be applied to any generally cylindrical implant to beinserted into a generally cylindrical bore. Generally cylindrical inthis context should be read as having parts coinciding with the envelopesurface of a cylinder circumscribing the implant. A screw-shaped implanthaving a fluted body (in similarity with a tap) thus for instance is tobe within the scope of the protection as conferred by the appendedclaims.

We claim:
 1. An implant fixture capable of being tapped or screwed intoa bore-hole in bone tissue, the fixture comprising:a body having atleast one part with a generally cylindrical surface; and threadsdisposed on the cylindrical surface, no thread having a height greaterthan about 0.20 mm.
 2. An implant fixture according to claim 1, whereinsaid threads have a height between 0.02 and 0.15 mm.
 3. An implantfixture according to claim 2, wherein said threads have a height between0.05 and 0.15 mm.
 4. An implant fixture according to claim 3, whereinsaid threads have a height of 0.10 mm.
 5. An implant fixture accordingto any one of the preceding claims, wherein adjacent threads areseparated by a crest-to-crest distance that is twice the height of thethreads.
 6. An implant fixture according to claim 5, wherein the threadsare multiple threads, preferably triple threads.
 7. An implant fixtureaccording to claim 5, wherein each thread has a flank and the anglebetween the flanks of the thread is 45°.
 8. An implant fixture accordingto claim 5, wherein each thread has a flank and the angle between theflanks of the thread is 60°.